Undervoltage protection circuit, undervoltage protection method and switching power supply

ABSTRACT

In one embodiment, an undervoltage protection circuit for a switching power supply can include: (i) an undervoltage detection circuit that determines whether an input voltage of the switching power supply is in an undervoltage state; (ii) a selection circuit configured to select a first or a second control signal to be provided as a main control signal to a control circuit; (iii) the control circuit being configured, in response to the main control signal being the first control signal, to control a switching operation of a power transistor in the switching power supply such that an output voltage of the switching power supply is maintained as substantially stable; and (iv) the control circuit being configured, in response to the main control signal being the second control signal, to control the switching operation of the power transistor to reduce an input power of the switching power supply.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201310097865.X, filed on Mar. 25, 2013, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of power supplies, and moreparticularly to an undervoltage protection circuit and method applied ina switching power supply.

BACKGROUND

Switch mode power supplies can efficiently convert electrical power froma source to a load, or to several different loads, with eachcorresponding to a different output. The main transistor of aswitching-mode supply can switch between on and off states at a givenoperating frequency, and voltage regulation can be achieved by varyingthe ratio of the on-to-off time of the main transistor. Switch modepower supplies may have relatively high power conversion efficiency, ascompared to other types of power converters. Switch mode power suppliesmay also be substantially smaller and lighter than a linear supply dueto the smaller transformer size and weight.

SUMMARY

In one embodiment, an undervoltage protection circuit configured for aswitching power supply, can include: (i) an undervoltage detectioncircuit configured to use a first voltage that represents an inputvoltage of the switching power supply to determine whether the inputvoltage is in an undervoltage state, and to output a detection result;(ii) a selection circuit configured to select a first or a secondcontrol signal to be provided as a main control signal to a controlcircuit based on the detection result, where the first control signalcomprises an output voltage error feedback signal of the switching powersupply, and the second control signal is less than the first controlsignal; (iii) the control circuit being configured, in response to themain control signal being the first control signal, to control aswitching operation of a power transistor in the switching power supplysuch that an output voltage of the switching power supply is maintainedas substantially stable; and (iv) the control circuit being configured,in response to the main control signal being the second control signal,to control the switching operation of the power transistor to reduce aninput power of the switching power supply.

In one embodiment, a method of controlling undervoltage protection in aswitching power supply, can include: (i) generating a detection resultby detecting whether an input voltage of the switching power supply isin an undervoltage state; (ii) selecting, based on the detection result,a first or a second control signal as a main control signal for acontrol circuit, where the first control signal comprises an outputvoltage error feedback signal of the switching power supply, and thesecond control signal is less than the first control signal; (iii)controlling, in response to the main control signal being the firstcontrol signal, a switching operation of a power transistor in theswitching power supply such that an output voltage of the switchingpower supply is maintained as substantially stable; and (iv)controlling, in response to the main control signal being the secondcontrol signal, the switching operation of the power transistor toreduce an input power of the switching power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first example undervoltageprotection circuit in accordance with embodiments of the presentinvention.

FIG. 2 is a schematic block diagram of an example undervoltageprotection circuit in accordance with embodiments of the presentinvention.

FIG. 3A is a schematic block diagram of a first example control circuitin the undervoltage protection circuit shown in FIG. 2, in accordancewith embodiments of the present invention.

FIG. 3B is a schematic block diagram of a second example control circuitin the undervoltage protection circuit shown in FIG. 2, in accordancewith embodiments of the present invention.

FIG. 3C is a schematic block diagram of a third example control circuitin the undervoltage protection circuit shown in FIG. 2, in accordancewith embodiments of the present invention.

FIG. 4 is a schematic block diagram of a second example undervoltageprotection circuit in accordance with embodiments of the presentinvention.

FIG. 5 is a flow diagram of an example method of undervoltageprotection, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

In various switching power supply applications, the external powersupply network may behave in an abnormal fashion (e.g., a sudden powerdown, insufficient voltage, etc.) in some cases. As a result, damage canoccur on a power device or transistor in the switching power supply.Therefore, in switching power supply design, undervoltage detection andprotection of the input voltage of the switching power supply may beaddressed.

In one approach, a sense voltage can be obtained via a series-connectedvoltage dividing resistor loop coupled to an external power networkvoltage. The sense voltage can change in proportion to the power networkvoltage, and when the sense voltage is reduced to less than apredetermined voltage, the system may take various protection measures.For example, such protection measures can include disabling theswitching power supply or employing a backup battery. Drawbacks of thisapproach to undervoltage detection and protection include when theexternal power supply voltage is relatively high, the dividing resistorsfor sensing are relatively large, and this may increase power losses aswell as overall system product costs.

In one embodiment, an undervoltage protection circuit configured for aswitching power supply, can include: (i) an undervoltage detectioncircuit configured to use a first voltage that represents an inputvoltage of the switching power supply to determine whether the inputvoltage is in an undervoltage state, and to output a detection result;(ii) a selection circuit configured to select a first or a secondcontrol signal to be provided as a main control signal to a controlcircuit based on the detection result, where the first control signalcomprises an output voltage error feedback signal of the switching powersupply, and the second control signal is less than the first controlsignal; (iii) the control circuit being configured, in response to themain control signal being the first control signal, to control aswitching operation of a power transistor in the switching power supplysuch that an output voltage of the switching power supply is maintainedas substantially stable; and (iv) the control circuit being configured,in response to the main control signal being the second control signal,to control the switching operation of the power transistor to reduce aninput power of the switching power supply.

Referring now to FIG. 1, shown is a schematic block diagram of a firstexample undervoltage protection circuit in accordance with embodimentsof the present invention. In this particular example, the undervoltageprotection circuit can be applied in a switching power supply (e.g., aflyback topology). Any suitable converter topology (e.g., forward,boost, buck, SEPIC, ZETA, etc.) can be utilized in particularembodiments. In this example, the flyback topology power stage circuitcan include a transformer (including primary winding T1 and secondarywinding T2), power transistor Q_(M), and series-connected samplingresistor R_(C).

As shown in FIG. 1, the input voltage information detection in thisparticular example can employ auxiliary inductor T3 that is coupled withan input inductor (e.g., primary winding T1) in the switching powersupply, so as to generate voltage V₁. Voltage V₁ can represent powersupply input voltage (e.g., V_(in)) information, according to an inducedvoltage on auxiliary inductor T3. In this way, by not directly using avoltage dividing resistor loop to sample the input voltage, chip pins(e.g., for external chip connections) and power losses for the overallcircuit may be reduced.

The undervoltage protection circuit can include undervoltage detectioncircuit 101, selection circuit 102, and control circuit 103.Undervoltage detection circuit 101 can receive voltage V₁ thatrepresents input voltage V_(in) of the switching power supply. Forexample, undervoltage detection circuit 101 can detect whether the inputvoltage is in an undervoltage state by comparing a peak signal ofvoltage V₁ against a reference voltage (e.g., V_(ref1), as shown in FIG.2) to generate detection result CTRL. The reference voltage for thiscomparison can be generated in undervoltage detection circuit 101, ormay be supplied by another circuit or component.

Selection circuit 102 can connect with undervoltage detection circuit101, and may select (e.g., based on detection result CTRL) eithercontrol signal V_(COMP) or control signal V_(C) _(_) _(min) to be usedas main control signal V_(C), and provided to control circuit 103. Thus,detection result CTRL can provide selection control to choose aparticular control signal for designation as “main” control signal V_(C)for control circuit 103. For example, control signal V_(COMP) caninclude an output voltage error feedback signal of the switching powersupply, and control signal V_(C) _(_) _(min) may be no greater than(e.g., less in absolute value than) control signal V_(COMP). In oneexample, control signal V_(C) _(_) _(min) may represent a minimum valueof control signal V_(COMP). However, control signal V_(C) _(_) _(min)may alternatively be set to other predetermined values.

Control circuit 103 can receive control signal V_(C) provided byselection circuit 102. When main control signal V_(C) is control signalV_(COMP), control circuit 103 can control the switching operation of apower transistor (e.g., Q_(M)) in the switching power supply such thatthe output voltage (e.g., at the load) of the switching power supplymaintained as substantially stable. This represents a normal operationof the switching power supply, such as when there is no undervoltagecondition detected. However, when main control signal V_(C) is controlsignal V_(C) _(_) _(min), control circuit 103 can control the switchingoperation of power transistor Q_(M) to reduce the input power of theswitching power supply. This represents an abnormal (e.g., undervoltage)condition of the switching power supply, and thus input power may bereduced as a protective measure.

In particular embodiments, an undervoltage protection circuit can detectwhether the input voltage of the switching power supply is in anundervoltage state via undervoltage detection circuit 101. If the inputvoltage is in a non-undervoltage state, the power stage circuit can becontrolled (e.g., via control circuit 103) to operate in a normalfashion. However, if the input voltage is in an undervoltage state, theinput power of the power stage circuit can be reduced so as to achieveundervoltage protection. As will be discussed in more detail below,particular embodiments can accommodate several approaches to reducingthe input power of the power stage circuit.

Referring now to FIG. 2, shown is a schematic block diagram of anexample undervoltage protection circuit in accordance with embodimentsof the present invention. In this example, undervoltage detectioncircuit 101 can include peak value detection circuit 201 and acomparison circuit (e.g., hysteresis comparator 202). Peak valuedetection circuit 201 can receive voltage V₁ that represents the inputvoltage of the switching power supply, and may generate peak voltagesignal V_(QPV) that represents peak information about the input voltage.Also, any suitable implementation of peak value detection circuit 201(see, e.g., Chinese Patent Application No. 201310076683.4) can besupported in particular embodiments.

Furthermore, hysteresis comparator 202 can receive peak voltage signalV_(QVP) and reference voltage V_(ref1), and may generate a comparisonoutput signal as detection result CTRL. For example, reference voltageV_(ref1) can be obtained by generating a voltage drop on resistor R₁ viacurrent source I₁. Specifically, when peak voltage signal V_(QPV) isless than reference voltage V_(ref1), the input voltage of the switchingpower supply can be detected as being in an undervoltage state, and thecomparison signal or detection result CTRL can be high. When peakvoltage signal V_(QPV) is greater than reference voltage V_(ref1),indicating that the input voltage of the switching power supply is in anon-undervoltage or normal operating state, the comparison signal ordetection result CTRL can be low.

When detection result CTRL is high, selection circuit 102 can selectcontrol signal V_(C) _(_) _(min) as main control signal V_(C) to beprovided to control circuit 103. When detection result CTRL is low,selection circuit 102 can select output voltage error feedback signalV_(COMP) as main control signal V_(C) to be provided to control circuit103. Here, output voltage error feedback signal V_(COMP) can be obtainedby comparing output voltage feedback voltage V_(fb) against referencevoltage V_(ref2), and by compensating the error amplifier signal via anRC circuit. For example, control signal V_(C) _(_) _(min) may be nogreater than (e.g., less in absolute value than) output voltage errorfeedback signal V_(COMP). In one particular example, control signalV_(C) _(_) _(min) can be a minimum value of output voltage errorfeedback signal V_(COMP).

Referring now to FIG. 3A, shown is a schematic block diagram of a firstexample control circuit in the undervoltage protection circuit shown inFIG. 2, in accordance with embodiments of the present invention. In thisparticular example, control circuit 103 can include a comparison circuit(e.g., comparator 301) and a logic circuit (e.g., RS flip-flop 302). Afirst input of comparator 301 can receive main control signal V_(C) fromselection circuit 102, and a second input of comparator 301 can receivesense voltage signal V_(S) that represents an input current of theswitching power supply.

Comparator 301 may provide comparison signal V_(C2) to reset terminal Rof RS flip-flop 302. Also, set terminal S of RS flip-flop 302 canreceive clock signal CLK1, and may generate logic control signal V_(Ct)for controlling the switching operation of power transistor Q_(M). Forexample, logic control signal V_(Ct) can be processed (e.g., buffered)by two inverters to enhance its driving ability for controlling theswitching operation of power transistor Q_(M).

Referring also to FIG. 2, when peak voltage signal V_(QPV) is less thanreference voltage V_(ref1), detection result CTRL can be high toindicate that the input voltage of the switching power supply is in anundervoltage state. In this case, selection circuit 102 can selectcontrol signal V_(C) _(_) _(min) as main control signal V_(C) to beprovided to control circuit 103. As shown in FIG. 3A, the invertinginput of comparator 301 in control circuit 103 can receive main controlsignal V_(C), and the non-inverting input can receive sense voltagesignal V_(S). When sense voltage signal V_(S) is higher than maincontrol signal V_(C), comparison signal V_(C2) output from comparator301 can go high. Because comparison signal V_(C2) is supplied to thereset terminal of RS flip-flop 302, logic control signal V_(Ct) can golow to turn off power transistor Q_(M). Thus in this case, the inputcurrent and thus the input power of the switching power supply can bereduced to address the undervoltage state.

When peak voltage signal V_(QPV) is greater than reference voltageV_(ref1), detection result CTRL can be low to indicate that the inputvoltage of the switching power supply is in a non-undervoltage state. Inthis case, selection circuit 102 can select output voltage errorfeedback signal V_(COMP) as main control signal V_(C) to be provided tocontrol circuit 103. As a result, control circuit 103 can control theoutput voltage of the switching power supply to be maintained assubstantially stable according to output voltage error feedback signalV_(COMP). Thus, the switching power supply can operate in a normalfashion when a non-undervoltage state is detected.

In this fashion, an undervoltage protection circuit can determinewhether the input voltage of a switching power supply is in anundervoltage state by comparing a peak voltage signal (e.g., V_(QPV))that indicates an input voltage peak against a reference voltage (e.g.,V_(ref1)). As compared to other approaches, product circuit area/volumeand cost can be reduced by avoiding a relatively large voltage dividingresistor loop. Also, when the input voltage is determined to be in theundervoltage state, the undervoltage protection circuit can reduce theinput power by reducing the input current. In this way, the power device(e.g., power transistor Q_(M)) in the switching power supply can beprotected from being damaged due to this undervoltage condition.

Referring now to FIG. 3B, shown is a schematic block diagram of a secondexample control circuit in the undervoltage protection circuit shown inFIG. 2, in accordance with embodiments of the present invention. In thisparticular example, clock signal regulating circuit 303 can be included.Clock signal regulating circuit 303 can receive main control signalV_(C) and clock signal CLK1. For example, clock signal regulatingcircuit 303 can regulate a frequency of clock signal CLK1 according tomain control signal V_(C). In this case, the regulated clock signal canbe denoted as CLK1′, and regulated clock signal CLK1′ can be provided tothe set terminal of RS flip-flop 302, as shown.

When the input voltage is detected as in the undervoltage state,selection circuit 102 can select control signal V_(C) _(_) _(min) asmain control signal V_(C) to be provided to control circuit 103. Controlcircuit 103 can reduce the input current of the switching power supplyaccording to control signal V_(C) _(_) _(min). Also, clock signalregulating circuit 303 can receive main control signal V_(C), and mayaccordingly reduce the frequency of clock signal CLK1 when controlsignal V_(C) _(_) _(min) is selected as main control signal V_(C). Thislowered frequency of the power transistor when the input voltage is inthe undervoltage state can reduce the input power of the switching powersupply, in order to realize undervoltage protection.

Referring now to FIG. 3C, shown is a schematic block diagram of a thirdexample control circuit in the undervoltage protection circuit shown inFIG. 2, in accordance with embodiments of the present invention. In thisparticular example, timing circuit 304 and OR-gate 305 can be added. Oneinput of timing circuit 304 can receive main control signal V_(C), andthe other input of timing circuit 304 can receive logic control signalV_(Ct). An output from timing circuit 304 can be provided as an input toOR-gate 305. OR-gate 305 can also receive comparison signal V_(C2), andmay provide an output to the reset terminal of RS flip-flop 302. Forexample, timing circuit 304 can regulate the on time of power transistorQ_(M) according to main control signal V_(C) and logic control signalV_(Ct).

When the input voltage is detected as being in the undervoltage state,selection circuit 102 can select control signal V_(C) _(_) _(min) asmain control signal V_(C) to be provided to control circuit 103. In thiscase, control circuit 103 can reduce the input current of the switchingpower supply according to control signal V_(C) _(_) _(min). Furthermore,timing circuit 304 can receive main control signal V_(C) and logiccontrol signal V_(Ct), and may accordingly regulate the on time of powertransistor Q_(M). For example, the on time (T_(ON)) of power transistorQ_(M) can be regulated to reduce the input power of the power stagecircuit to address the undervoltage condition, thus realizingundervoltage protection.

In this fashion, when the input voltage of the switching power supply isin the undervoltage state, the undervoltage protection circuit ofparticular embodiments can thus employ different protection methods. Forexample, the input current of the switching power supply can be reduced(e.g., via the control circuit of FIG. 3A), the operating frequency ofthe power transistor reduced (e.g., via the control circuit of FIG. 3B),and/or the on time of the power transistor can be reduced (e.g., via thecontrol circuit of FIG. 3C). In any event, the input power of the powerstage circuit in the switching power supply can be reduced so as toprotect a power device (e.g., transistor Q_(M)) in the switching powersupply from being damaged due to the undervoltage condition.

Referring now to FIG. 4, shown is a schematic block diagram of a secondexample undervoltage protection circuit in accordance with embodimentsof the present invention. In this particular example, the undervoltageprotection circuit can include auxiliary voltage circuit 401,discharging circuit 402, and load detection circuit 403. Here, auxiliaryvoltage circuit 401 can receive voltage V₁ that represents the inputvoltage of the switching power supply, and may generate power supplyvoltage V_(CC). For example, power supply voltage V_(CC) can beconfigured to supply voltage for the control chip (e.g., includingcontrol circuit 103) of the switching power supply. Load detectioncircuit 403 can be configured to detect a load state (e.g., an unloadedstate, a loaded state, a light load, a heavy load, etc.) of theswitching power supply.

Discharging circuit 402 can receive detection result CTRL and powersupply voltage V_(CC), and may determine whether to discharge powersupply voltage V_(CC) according to detection result CTRL and the loadstate. Discharging circuit 402 can include transistor Q₁ and resistorR₂. A first power terminal of transistor Q₁ can receive power supplyvoltage V_(CC), a second power terminal of transistor Q₁ can be groundedvia series-connected resistor R₂, and a control terminal of transistorQ₁ can receive (e.g., via an AND-gate) detection result CTRL. Forexample, when detection result CTRL is high, and load detection circuit403 detects that the load is unloaded, transistor Q₁ can be turned on,and power supply voltage V_(CC) can be discharged through resistor R₂ sothat the control chip (e.g., control circuit 103) may be disabled toreduce the associated power losses.

In one embodiment, a method of controlling undervoltage protection in aswitching power supply, can include: (i) generating a detection resultby detecting whether an input voltage of the switching power supply isin an undervoltage state; (ii) selecting, based on the detection result,a first or a second control signal as a main control signal for acontrol circuit, where the first control signal comprises an outputvoltage error feedback signal of the switching power supply, and thesecond control signal is less than the first control signal; (iii)controlling, in response to the main control signal being the firstcontrol signal, a switching operation of a power transistor in theswitching power supply such that an output voltage of the switchingpower supply is maintained as substantially stable; and (iv)controlling, in response to the main control signal being the secondcontrol signal, the switching operation of the power transistor toreduce an input power of the switching power supply.

Referring now to FIG. 5, shown is a flow diagram of an example method ofundervoltage protection, in accordance with embodiments of the presentinvention. At 502, whether an input voltage of a switching power supplyis in an undervoltage state can be detected (e.g., via undervoltagedetection circuit 101), and a detection result (e.g., CTRL) can beaccordingly output. At 504, a first or second control signal can beselected (e.g., via selection circuit 102) to be provided as a maincontrol signal to a control circuit (e.g., 103) according to thedetection result. For example, the first control signal can include anoutput voltage error feedback signal (e.g., V_(COMP)) of the switchingpower supply, and the second control signal (e.g., V_(C) _(_) _(min))can be no greater than (e.g., a minimum of) the first control signal.

At 506, when the main control signal is the first control signal, theswitching operation of a power transistor (e.g., Q_(M)) in the switchingpower supply can be controlled such that an output voltage of theswitching power supply is maintained as substantially stable. Thisrepresents the normal operation of the switching power supply. At 508,when the main control signal is the second control signal, the switchingoperation of the power transistor can be controlled to reduce the inputpower of the switching power supply. This represents undervoltageprotection operation of the switching power supply. Also, as discussedabove, various approaches to lowering the input power (e.g., loweringinput current, reducing operating frequency, reducing transistor ontime) can be supported in particular embodiments.

The example method can also include receiving a voltage (e.g., V₁) thatrepresents the input voltage (e.g., V_(in)) of the power supply voltage,and generating a power supply voltage (e.g., V_(CC)). The load state ofthe switching power supply can also be detected, and a determination asto whether to discharge (e.g., via transistor Q₁ and resistor R₂) thepower supply voltage can be made according to the detection result andthe load state.

Particular embodiments can also include a switching power supply, whichcan include a power stage circuit and an undervoltage protectioncircuit, as described herein. The power stage circuit can be anyappropriate converter topology structure (e.g., flyback, forward, boost,buck, SEPIC, ZETA, etc.). Similarly, the switching power supply, by useof the above-described undervoltage protection circuitry, can protectone or more power transistors therein from being damaged as a result ofan undervoltage condition.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to the particularuse contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

1. An undervoltage protection circuit comprising: a) an undervoltagedetection circuit configured to determine whether an input voltage of aswitching power supply is in an undervoltage state, and to output adetection result; b) a selection circuit configured to select a first ora second control signal to be provided as a main control signal based onsaid detection result, wherein said second control signal represents aminimum value of said first control signal; c) a control circuitconfigured, in response to said main control signal being said firstcontrol signal, to control said switching power supply in a normaloperating state; and d) said control circuit being configured, inresponse to said main control signal being said second control signal,to control said switching power supply in an undervoltage protectionoperating state.
 2. The undervoltage protection circuit of claim 1,wherein said undervoltage detection circuit comprises: a) a peak valuedetection circuit configured to receive a first voltage, and togenerates a peak voltage signal that represents peak information of saidinput voltage; and b) a comparison circuit configured to receive saidpeak voltage signal and a first reference voltage, and to generate afirst comparison signal as said detection result, wherein said firstcomparison signal is high when said peak voltage signal is less thansaid first reference voltage to indicate that said input voltage is inan undervoltage state, and wherein said first comparison signal is lowwhen said peak voltage signal is greater than said first referencevoltage to indicate that said input voltage is in a non-undervoltagestate.
 3. The undervoltage protection circuit of claim 2, wherein saidselection circuit is configured to: a) select said first control signalas said main control signal when said first comparison signal is low;and b) select said second control signal as said main control signalwhen said first comparison signal is high.
 4. The undervoltageprotection circuit of claim 1, wherein said control circuit comprises:a) a second comparison circuit configured to receive said main controlsignal and a sense voltage signal that represents an input current ofsaid switching power supply, and to generate a second comparison signal;and b) a logic circuit configured to receive said second comparisonsignal and a first clock signal, and to generate a logic control signalto control said switching operation of said power transistor.
 5. Theundervoltage protection circuit of claim 4, wherein said control circuitfurther comprises a clock signal regulating circuit configured toregulate a frequency of said first clock signal based on said maincontrol signal.
 6. The undervoltage protection circuit of claim 4,wherein said control circuit further comprises: a) a timing circuitconfigured to regulate an on time of said power transistor based on saidmain control signal and said logic control signal; and b) an OR-gateconfigured to receive an output from said timing circuit and said secondcomparison signal, and to provide an output signal to said logiccircuit.
 7. The undervoltage protection circuit of claim 1, wherein saidundervoltage protection circuit further comprises: a) an auxiliary powersupply circuit comprising an auxiliary winding coupled with an inputinductor of said switching power supply, being configured to receive afirst voltage, and to generate a power supply voltage; b) a loaddetection circuit configured to detect a load state of said switchingpower supply; and c) a discharging circuit configured to receive saiddetection result and said power supply voltage, and to determine whetherto discharge said power supply voltage according to said detectionresult and said load state.
 8. A method, comprising: a) generating adetection result by detecting whether an input voltage of said aswitching power supply is in an undervoltage state; b) selecting, basedon said detection result, a first or a second control signal as a maincontrol signal, wherein said second control signal represents a minimumvalue of said first control signal; c) controlling, in response to saidmain control signal being said first control signal, said switchingpower supply in a normal operating state; and d) controlling, inresponse to said main control signal being said second control signal,said switching power supply in an undervoltage protection operatingstate.
 9. The method of claim 8, further comprising: a) generating apower supply voltage from a first voltage; b) detecting a load state ofsaid switching power supply; and c) determining whether to dischargesaid power supply voltage based on said detection result and said loadstate.
 10. A switching power supply, comprising: a) a power stagecircuit; and b) said undervoltage protection circuit of claim
 1. 11. Theswitching power supply of claim 10, wherein said power stage circuitcomprises a flyback converter topology.
 12. The switching power supplyof claim 10, wherein said power stage circuit comprises a boostconverter topology.
 13. The switching power supply of claim 10, whereinsaid power stage circuit comprises a buck converter topology.
 14. Theundervoltage protection circuit of claim 2, wherein said first voltageis at a common node of an auxiliary winding and an anode of a diode. 15.The undervoltage protection circuit of claim 1, wherein said controlcircuit is configured to receive a sense voltage that represents aninput current of said switching power supply.
 16. The undervoltageprotection circuit of claim 15, wherein said control circuit isconfigured to control said switching power supply based on said maincontrol signal and said sense voltage.
 17. The method of claim 8,further comprising receiving a sense voltage that represents an inputcurrent of said switching power supply.
 18. The method of claim 17,further comprising controlling said switching power supply based on saidmain control signal and said sense voltage.
 19. The method of claim 8,wherein said generating said detection result comprises generating, by apeak value detection circuit, a peak voltage signal that represents peakinformation of said input voltage.
 20. The method of claim 19, whereinsaid generating said detection result comprises comparing said peakvoltage signal against a first reference voltage.